The technique of high resolution mass spectrometry will be used to analyze for base modifications in DNA induced by ionizing radiation and reduced oxygen species. Normal and irradiated calf thymus DNA will be treated by conventional thermal techniques or by the field description technique in an attempt to release and vaporize base residues for subsequent ionization and analysis by mass spectrometry. It is quite possible that damaged bases can be released from irradiated DNA much more readily than unmodified bases. The spectra obtained will be compared with those of irradiated and non-irradiated bases. The spectra obtained will be compared with those of irradiated and non-irradiated purine and pyrimidine bases. Several characteristic fragmentation reactions should provide a means for identifying base modifications. This approach should allow us to determine the structure of modified base residues that result from treatment of DNA with ionizing radiation or reduced oxygen species.